函数中提前收益的有效性

The short answer is, no difference. Do yourself a favour and stop worrying about this. The optimising compiler is almost always smarter than you.

Concentrate on readability and maintainability.

If you want to see what happens, build these with optimisations on and look at the assembler output.

Even though this isn't much an answer, a production compiler is going to be much better at optimizing than you are. I would favor readability and maintainability over these kinds of optimizations.

If you really want to know if there's a difference in compiled code for your particular compiler and system, you'll have to compile and look at the assembly yourself.

However in the big scheme of things it's almost certain that the compiler can optimize better than your fine tuning, and even if it can't it's very unlikely to actually matter for your program's performance.

Instead, write the code in the clearest way for humans to read and maintain, and let the compiler do what it does best: Generate the best assembly it can from your source.

I agree strongly with blueshift: readability and maintainability first!. But if you're really worried (or just want to learn what your compiler is doing, which definitely a good idea in the long run), you should look for yourself.

This will mean using a decompiler or looking at low level compiler output (e.g. assembly lanuage). In C#, or any .Net language, the tools documented here will give you what you need.

But as you yourself have observed, this is probably premature optimization.

To be specific about this, the return will be compiled into a branch to the end of the method, where there will be a RET instruction or whatever it may be. If you leave it out, the end of the block before the else will be compiled into a branch to the end of the else block. So you can see in this specific case it makes no difference whatsoever.

There is no difference at all:

=====> cat test_return.cpp
extern void something();
extern void something2();


void test(bool b)
{
if(b)
{
something();
}
else
something2();
}
=====> cat test_return2.cpp
extern void something();
extern void something2();


void test(bool b)
{
if(b)
{
something();
return;
}
something2();
}
=====> rm -f test_return.s test_return2.s
=====> g++ -S test_return.cpp
=====> g++ -S test_return2.cpp
=====> diff test_return.s test_return2.s
=====> rm -f test_return.s test_return2.s
=====> clang++ -S test_return.cpp
=====> clang++ -S test_return2.cpp
=====> diff test_return.s test_return2.s
=====>

Meaning no difference in generated code whatsoever even without optimization in two compilers

Interesting answers: Although I do agree with all of them (so far), there are possible connotations to this question that are up to now completely disregarded.

If the simple example above is extended with resource allocation, and then error checking with a potential resulting freeing of resources, the picture might change.

Consider the naive approach beginners might take:

int func(..some parameters...) {
res_a a = allocate_resource_a();
if (!a) {
return 1;
}
res_b b = allocate_resource_b();
if (!b) {
free_resource_a(a);
return 2;
}
res_c c = allocate_resource_c();
if (!c) {
free_resource_b(b);
free_resource_a(a);
return 3;
}


do_work();


free_resource_c(c);
free_resource_b(b);
free_resource_a(a);


return 0;
}

The above would represent an extreme version of the style of returning prematurely. Notice how the code becomes very repetitive and non-maintainable over time when its complexity grows. Nowadays people might use exception handling to catch these.

int func(..some parameters...) {
res_a a;
res_b b;
res_c c;


try {
a = allocate_resource_a(); # throws ExceptionResA
b = allocate_resource_b(); # throws ExceptionResB
c = allocate_resource_c(); # throws ExceptionResC
do_work();
}
catch (ExceptionBase e) {
# Could use type of e here to distinguish and
# use different catch phrases here
# class ExceptionBase must be base class of ExceptionResA/B/C
if (c) free_resource_c(c);
if (b) free_resource_b(b);
if (a) free_resource_a(a);
throw e
}
return 0;
}

Philip suggested, after looking at the goto example below, to use a break-less switch/case inside the catch block above. One could switch(typeof(e)) and then fall through the free_resourcex() calls but this is not trivial and needs design consideration. And remember that a switch/case without breaks is exactly like the goto with daisy-chained labels below...

As Mark B pointed out, in C++ it is considered good style to follow the Resource Aquisition is Initialization principle, RAII in short. The gist of the concept is to use object instantiation to aquire resources. The resources are then automatically freed as soon as the objects go out of scope and their destructors are called. For interdepending resources special care has to be taken to ensure the correct order of deallocation and to design the types of objects such that required data is available for all destructors.

Or in pre-exception days might do:

int func(..some parameters...) {
res_a a = allocate_resource_a();
res_b b = allocate_resource_b();
res_c c = allocate_resource_c();
if (a && b && c) {
do_work();
}
if (c) free_resource_c(c);
if (b) free_resource_b(b);
if (a) free_resource_a(a);


return 0;
}

But this over-simplified example has several drawbacks: It can be used only if the allocated resources do not depend on each other (e.g. it could not be used for allocating memory, then opening a filehandle, then reading data from the handle into the memory), and it does not provide individial, distinguishable error codes as return values.

To keep code fast(!), compact, and easily readable and extensible Linus Torvalds enforced a different style for kernel code that deals with resources, even using the infamous goto in a way that makes absolutely sense:

int func(..some parameters...) {
res_a a;
res_b b;
res_c c;


a = allocate_resource_a() || goto error_a;
b = allocate_resource_b() || goto error_b;
c = allocate_resource_c() || goto error_c;


do_work();


error_c:
free_resource_c(c);
error_b:
free_resource_b(b);
error_a:
free_resource_a(a);


return 0;
}

The gist of the discussion on the kernel mailing lists is that most language features that are "preferred" over the goto statement are implicit gotos, such as huge, tree-like if/else, exception handlers, loop/break/continue statements, etc. And goto's in the above example are considered ok, since they are jumping only a small distance, have clear labels, and free the code of other clutter for keeping track of the error conditions. This question has also been discussed here on stackoverflow.

However what's missing in the last example is a nice way to return an error code. I was thinking of adding a result_code++ after each free_resource_x() call, and returning that code, but this offsets some of the speed gains of the above coding style. And it's hard to return 0 in case of success. Maybe I'm just unimaginative ;-)

So, yes, I do think there is a big difference in the question of coding premature returns or not. But I also think it is apparent only in more complicated code that is harder or impossible to restructure and optimize for the compiler. Which is usually the case once resource allocation comes into play.

In your example, the return is noticeable. What happens to the person debugging when the return is a page or two above/below where //do different stuff occurs? Much harder to find/see when there is more code.

void foo1(bool flag)
{
if (flag)
{
//Do stuff
return;
}


//Do different stuff
}


void foo2(bool flag)
{
if (flag)
{
//Do stuff
}
else
{
//Do different stuff
}
}

One school of thought (can't remember the egghead who proposed it at the moment) is that all function should only have one return point from a structural point of view to make the code easier to read and debug. That, I suppose, is more for programming religious debate.

One technical reason you may want to control when and how a function exits that breaks this rule is when you are coding real-time applications and you want to make sure that all control paths through the function take the same number of clock cycles to complete.

From Clean Code: A Handbook of Agile Software Craftsmanship

Flag arguments are ugly. Passing a boolean into a function is a truly terrible practice. It immediately complicates the signature of the method, loudly proclaiming that this function does more than one thing. It does one thing if the flag is true and another if the flag is false!

foo(true);

in code will just make the reader to navigate to the function and waste time reading foo(boolean flag)

Better structured code base will give you better opportunity to optimize code.

I'm glad you brought this question up. You should always use the branches over an early return. Why stop there? Merge all your functions into one if you can (at least as much as you can). This is doable if there is no recursion. In the end, you will have one massive main function, but that is what you need/want for this sort of thing. Afterward, rename your identifiers to be as short as possible. That way when your code is executed, less time is spent reading names. Next do ...